Propargyl radical:: Ab initio anharmonic modes and the polarized infrared absorption spectra of matrix-isolated HCCCH2

The propargyl radical has twelve fundamental vibrational modes, Gamma(vib)(HCCCH2) = 5a(1) circle plus 3b(1) circle plus 4b(2), and nine have been detected in a cryogenic matrix. Ab initio coupled-cluster anharmonic force field calculations were used to help guide some of the assignments. The experimental HC = C - CH2 matrix frequencies (cm(-1)) and polarizations are a(1) modes-3308.5 +/- 0.5, 3028.3 +/- 0.6, 1935.4 +/- 0.4, 1440.4 +/- 0.5, 1061.6 +/- 0.8; b(1) modes-686.6 +/- 0.4, 483.6 +/- 0.5; b(2) modes-1016.7 +/- 0.4, 620 +/- 2. We recommend a complete set of gas-phase vibrational frequencies for the propargyl radical, HC = C - CH2 (X) over tilde B-2(1). From an analysis of the vibrational spectra, the small electric dipole moment, mu(D)(HCCCH2) = 0.150 D, and the large resonance energy (HCCCH2), roughly 11 kcal mol(-1), we conclude that propargyl is a completely delocalized hydrocarbon radical and is best written as HC = C - CH2.